Micas are commonly used in Ar-40/Ar-39 thermochronological studies of
variably deformed rocks yet the physical basis by which deformation may
affect radiogenic argon retention in mica is poorly constrained. This
study examines the relationship between deformation and
deformation-induced microstructures on radiogenic argon retention in
muscovite, A combination of furnace step-heating and high-spatial
resolution in situ UV-laser ablation Ar-40/Ar-39 analyses are reported
for deformed muscovites sampled from a granitic pegmatite vein within
the Siviez-Mischabel Nappe, western Swiss Alps (Penninic domain,
Brianconnais unit). The pegmatite forms part of the Variscan (similar to
350 Ma) Alpine basement and exhibits a prominent Alpine S-C fabric
including numerous mica `fish' that developed under greenschist facies
metamorphic conditions, during the dominant Tertiary Alpine tectonic
phase of nappe emplacement. Furnace step-heating of milligram quantities
of separated muscovite grains yields an Ar-40/Ar-39 age spectrum with
two distinct staircase segments but without any statistical plateau,
consistent with a previous study from the same area. A single (3 X 5 mm)
muscovite porphyroclast (fish) was investigated by in situ UV-laser
ablation. A histogram plot of 170 individual Ar-40/Ar-39 UV-laser
ablation ages exhibit a range from 115 to 387 Ma with modes at
approximately 340 and 260 Ma. A variogram statistical treatment of the
(40)Ad/Ar-39 results reveals ages correlated with two directions; a
highly correlated direction at 310 degrees and a lesser correlation at 0
degrees relative to the sense of shearing. Using the highly correlated
direction a statistically generated (Kriging method) age contour map of
the Ar-40/Ar-39 data reveals a series of elongated contours subparallel
to the C-surfaces which where formed during Tertiary nappe emplacement.
Similar data distributions and slightly younger apparent ages are
recognized in a smaller mica fish. The observed intragrain age
variations are interpreted to reflect the partial loss of radiogenic
argon during Alpine (similar to 35 Ma) greenschist facies metamorphism.
One-dirnensional diffusion modelling results are consistent with the
idea that the zones of youngest apparent age represent incipient shear
band development within the mica porphyroclasts, thus providing a
network of fast diffusion pathways. During Alpine greenschist facies
metamorphism the incipient shear bands enhanced the intragrain loss of
radiogenic argon. The structurally controlled intragrain age variations
observed in this investigation imply that deformation has a direct
control on the effective length scale for argon diffusion, which is
consistent with the heterogeneous nature of deformation. (C) 2001
Elsevier Science B.V. All rights reserved.